1. Field Of The Invention
This invention relates to aromatic vinyl ether compounds and compositions which are cationically polymerizable in the presence of a suitable photoinitiator under photopolymerization conditions, and also relates to a method of making such aromatic vinyl ether compounds and compositions.
2. Description of The Related Art
In the field of radiation-cured materials, for applications such as adhesives, sealants, and coatings, significant effort has been directed to developing cationically polymerizable, radiation-curable materials. Such interest in cationically curable materials is a consequence of the potential advantages of such compositions over conventional free radical-initiated radiation-curable materials, such as (meth)acrylates.
Currently, there are three primary categories of radiation curable systems--(meth)acrylates, thiol-enes, and epoxies. The most popular of these systems is the (meth)acrylate-based UV free radical curing system, for which a wide variety of suitable prepolymers is commercially available. These (meth)acrylate-based UV curing systems, however, suffer from deficiencies associated with the odor and toxicity of acrylate monomers, poor adhesion, and oxygen inhibition effects.
Thiol-ene UV free-radical curing systems have been developed, such as the thio/norbornene systems reported in Jacobine, A., Polymeric Materials Sci. Eng., 60, 211. 1989 These systems yield flexible coatings, but have the disadvantages of thiol odor and poor adhesion characteristics.
Epoxy-based UV cationically curable systems are known. Since the development of UV cationic initiators in the 1970's (see Crivello, J. V., et al, Macromolecules, 10, 1307 (1977)), UV cationically curable systems have attracted increasing interest and have gained in popularity as a radiation cure technology.
Relative to free-radical initiated, radiation-curing systems, UV cationic cure systems have the advantages of higher curing speeds, lack of oxygen inhibition, dark curing (i.e., continuation of cure even after cessation of UV radiation exposure), superior shelf-stability, lower toxicity, and lower odor characteristics.
Despite their many advantages, however, UV cationic cure systems technology has been hindered by limited availability of commercially useful monomers, and associated high costs of those monomers which are commercially available.
The development of epoxy-based UV cationically curing compositions has met with limited success. One of the most versatile and widely employed epoxy resins is diglycidyl ether of Bisphenol A (DGEBPA). Despite its widespread and successful use in non-cationic curing systems, DGEBPA and numerous other epoxy resins are characterized by unsuitably low curing rates in UV cationic curing systems. At present, the only widely available epoxy materials which are suitable for use in rapid UV cationically curing systems, are alicyclic diepoxides (see Koleske, J. V., RadTech '88 --North American Conference Papers, 353 (1988)), such as ##STR2## which is commercially available under the trademark Cyracure UVR 6100 (Union Carbide Corporation; Danbury, Conn.).
It is known that vinyl ether monomers are readily polymerizable by cationic polymerization, and a variety of vinyl ether monomers have been synthesized, however, prior to the 1970's, cationic polymerization vinyl ether technology was not practical due to severe polymerization conditions, e.g., polymerization reaction temperatures on the order of -70.degree. C., which were required to cationically polymerize vinyl ether monomers. Beginning in the mid-1970's, UV photocationic initiators were developed, which initiate the polymerization of vinyl ether monomers at room temperature conditions.
As a result of the development of UV cationic initiators, UV cationically curable vinyl ether and epoxy systems have attracted interest, and some vinyl ether and epoxy cationically curable systems are now commercially available. As indicated, however, the number of such monomers is limited and the costs of such monomers are significant in comparison to monomers employed in free-radical initiated polymerization systems.
Commercially available vinyl ether monomers which are curable by UV cationic cure methods (see Dougherty, J. A., et al, RadTech '88--North American Conference Papers. 372 (1988)) include vinyl ether monomers of the formulae: ##STR3## These vinyl ether monomers are cationically curable at much faster rates than the commercially available cationically curable epoxy systems. Since these vinyl ether compounds have aliphatic (Formula 1) and cycloaliphatic (Formula 2) backbone structures, respectively, their glass transition temperatures (T.sub.g 's) are so low (20.degree. C. and 80.degree. C., respectively) that they are practically useful only as reactive diluents for UV cationic curing epoxy systems.
In order to overcome the problems associated with such low T.sub.g values of the above-described vinyl ether monomers, divinyl ether monomers possessing an aromatic backbone structure have been developed, e.g., the bisphenol backbone monomer of the formula: ##STR4##
The problem with such type of vinyl ether compounds is that the urethane linkages ##STR5## significantly slow the UV cationic curing rate of the monomer. Although the urethane moiety is not as strong a base as an amine or other active hydrogen species, the urethane group acts as a weak counter anion in cationic polymerization systems, thereby retarding the rate of cationic polymerization of the vinyl ether monomers. In other, less reactive UV cationically curable monomers systems, such as those comprising alicyclic epoxies, the presence of urethane groups completely inhibits the UV cationic cure.
In an effort to overcome the slow or non-curing characteristics of urethane-containing vinyl ether monomers, vinyl ether compounds have been developed which contain only ether linkages, such as the Bisphenol A-based vinyl ether compound of the formula: ##STR6## The problem with vinyl ether compounds of such type is that the aromatic vinyl ether rearranges to yield only low molecular weight polymers containing pendant phenolic groups, as shown in the following model reaction (see Crivello, J. V., J. Polymer Sci., Polymer Chem. Ed., 21, 1785 (1983)): ##STR7## Such rearrangement problem was overcome by incorporation of an alkylene (ethylene) group between the phenoxy and vinyloxy moieties of the aromatic vinyl ether molecule in Bisphenol A-based compounds such as: ##STR8## This compound contains a rigid aromatic (Bisphenol A residue) backbone which is similar in structure to the backbone of DGEBPA epoxy resins. As a result, the cured properties of such vinyl ether compound, such as flexural modulus, heat distortion temperature, and glass transition temperature, are close to the cured properties of DGEBPA. In addition, this aromatic vinyl ether compound cures significantly faster than the corresponding epoxy system, e.g., a composition comprising an alicyclic epoxy such as 4-vinylcyclohexene dioxide, and DGEBPA. The Bisphenol A-based divinyl ether compound is an excellent resin, but unfortunately suffers a major deficiency in that it is a solid at room temperature (with a melting point on the order of 55.degree.-57.degree. C.), and thus cannot be satisfactorily used in adhesive and coating applications.
Thus, aromatic backbones such as Bisphenol A-based structures are essential to provide adequate rigidity and toughness in the cured vinyl ether resin. The majority of divinyl either compounds containing an aromatic backbone structure, however, are solids at room temperature and have melting points substantially above 25.degree. C. (See Crivello, J. V., J. Polymer Sci., Polymer Chem. Ed., 21 1785 (1983).
In an effort to obtain liquid divinyl ether monomers containing an aromatic backbone structure, chain-extended compounds have been synthesized as reaction products of aromatic divinyl ether monomers with active hydrogen compounds such as aryl polyols, e.g., Bisphenol A, to yield liquid reaction products. These chain-extended compounds cure to form products which are less brittle than the polymerized divinyl ethers obtained from non-chain-extended monomers, due to the lower cross-linking density achievable with the chain-extended compounds. Unfortunately, the chain-extended compounds are characterized by high viscosity and by the presence of acetal linkages which are hydrolytically unstable, resulting in the undesired formation of formaldehyde, phenol, and hydroxy-functional reaction products.
U.S. Pat. No. 3,803,246 issued Apr. 9, 1974 to K. S. Rosenzweig, et al. discloses the formation of oxyalkylated diphenol compositions by reacting diphenol with an alkylene oxide in the presence of lithium hydroxide or lithium acetate. The patent notes at column 1, lines 48-55 that when the corresponding diphenol/alkylene oxide reaction is carried out in the presence of sodium hydroxide, the product contains, in addition to alkoxylated diphenols, substantial amounts of oxyalkylated decomposition products of diphenol, such as oxyalkylated phenol and oxyalkylated isopropenyl phenol. These diphenol decomposition products are said to be substantially reduced by the use of the lithium hydroxide or lithium acetate constituent. The patent at column 2, lines 50-52 also discusses reacting the product oxyalkylated diphenol with additional alkylene oxide in the presence of a "non-lithium containing alkoxylation catalyst." The patent lists various diphenols which may be employed to form the oxyalkylated diphenol product, including bisphenol compounds wherein the aromatic moiety comprises a methyl substitutent on the ring. The patent discloses to form polyesters of allegedly improved properties, by esterifying the oxyalkylated diphenol compounds with dicarboxylic acids or anhydrides. The patent discloses that at least 80 mole percent of the dicarboxylic acid or anhydride preferably is ethylenically unsaturated dicarboxylic acid or anhydride. Example VII of the patent discloses the reaction of 2,2-bis(4-hydroxy-3-methyl-phenyl) propane with ethylene oxide in the presence of lithium hydroxide monohydrate, with a resulting product then being reacted with ethylene oxide in the presence of sodium hydroxide to yield polyoxyethylene (14) 2,2-bis(4-hydroxy-3-methyl-phenyl) propane. The patent at column 2, lines 20-30 discloses the diphenol reactant as being substitutable on the phenylene rings with C.sub.1 -C.sub.4 alkyl.
U.S. Pat. No. 4,603,162 issued July 29, 1986 to Y. Hasegawa, et al. describes a radiation curable resin of the formula: ##STR9## wherein R is a methylol group or (meth)acrylated methylol group. The molar ratio of methylol to (meth)acrylated methylol substituents is preferably in the range of 20:1 to 1:3. The resin is formed by reacting Bisphenol A with formaldehyde in the molar ratio of 1:4, to yield tetramethylol Bisphenol A, and reacting the methylolated product with acrylic acid, methyacrylic acid, or a lower alcohol ester thereof, yielding the (meth)acrylated product.
U.S. Pat. No. 4,622,376 issued Nov. 11, 1986 to M. S. Misura, et al. discloses a pourable polymerizable composition comprising (1) an aromatic-containing poly(allyl carbonate)-functional material comprising aromatic-containing bis(allyl carbonate)-functional monomer and/or aromatic-containing poly(allyl carbonate)-functional polymer, (2) styrenic material, and (3) polyethylenic-functional monomer containing three or more allyl, methallyl, acrylyl and/or methacrylyl groups. The patent discloses in the paragraph bridging columns 2 and 3 of the patent that the aromatic-containing bis(allyl carbonate)-functional monomers comprise a divalent radical derived from a dihydroxy aromatic-containing material, which may be a polyol-functional chain-extended compound such as an alkylene oxide extended bisphenol (column 3, lines 21-23). The reference discloses that the aromatic rings in such bisphenol may include substituents such as C.sub.1 -C.sub.4 alkyl, phenyl, or halo. At column 7, lines 3-27, the patent discloses that a wide variety of compounds may be used as the polyethylenic functional monomer containing three or more allyl, methyallyl, acrylyl, and/or methacrylyl groups.
U.S. Pat. No. 4,439,291 issued Mar. 27, 1984 to E. Irving, et al. describes a polymerizable composition including (1) a compound containing (a) at least one acryloyloxy and/or methacryloyloxy group, and (b) at least one allyl, methallyl, and/or 1-propenyl group attached directly to a carbon atom which forms part of an aromatic nucleus or to an oxygen atom or oxycarbonyl group which in turn is directly attached to such a carbon atom, in which the total number of groups (a) and (b) is at least three, and (2) a compound containing at least two mercaptan groups directly attached to aliphatic carbon atoms, in in a specified proportion of mercaptan groups to the other specified functional groups. Among the compounds (1) disclosed in the patent specification are: 2,2-bis(3-allyl-4-(methacryloyloxy)phenyl) propane [column 5, line 49]; and the corresponding methallyl compound [column 5, line 51]. The polymerizable composition disclosed in this reference may contain a photosensitizing agent and a heat-activated free-radical catalyst for two-stage curing, comprising initial exposure to actinic radiation followed by heating, to form cured compositions useful in applications such as coatings, adhesives, and reinforced composites. In its "Background" section, this patent identifies U.S. Pat. No. 4,220,513 as disclosing 2,2-bis(3-allyl-4- (glycidyloxy)phenyl) propane, and U.S. Pat. No. 4,308,367 is identified as disclosing 2,2-bis(3-allyl-4-hydroxyphenyl)propane. Various 2,2-bis(allyl-((meth)acryloxyalkoxy)phenyl)propane compounds are described in the patent; see, for example formula XXXIV at column 20 of the specification, and formula XXXVIII at column 22 thereof.
U.S. Pat. No. 4,683,327 issued July 28, 1987 to R. W. Stackman describes heat-curable acrylic-terminated aromatic monomers wherein the monomer backbone comprises oxycarbonyl groups alternating with divalent aromatic radicals, and wherein the backbone may comprise C.sub.1 -C.sub.4 alkoxy groups. The patent specification in the sentence bridging columns 4 and 5 discloses that the hydrogen atoms present on the aromatic rings may be replaced by alkyl, alkoxy, halogen, phenyl, and substituted phenyl substituents.
U.S. Pat. No. 4,388,450 issued June 14, 1983 to J. V. Crivello describes cationically curable compositions comprising aromatic polyvinylethers, such as 2,2-bis(p-vinyloxy- ethoxyphenyl) propane, and "reaction products of such materials with various active hydrogen compounds, for example, polycarboxylic acids, phenols, silanes, thiols, etc." (column 2, lines 5-8). The thermal curing catalyst employed with such compositions comprises an aryl/onium salt, and an organic oxidant or an aromatic polyvinylether-soluble copper compound. A wide variety of vinyloxyalkoxy aromatic compounds are disclosed. The patent as indicated teaches to further react the aromatic polyvinylether compound with a phenolic reactant such as Bisphenol A (column 6, lines 30-35). The patent discloses to make the aromatic vinyl ether compounds by condensing an alkali metal-aryl hydroxide or acid salt with a haloalkylvinylether in the presence of dimethylsulfoxide.
At column 8, lines 26-30, the patent discloses that the curable composition may be formed by melt-blending:
"Depending on upon the particular ingredients used in the heat curable mixture, melt blending can be used in certain instances, particularly where the aromatic polyvinylether has a melting point of 25.degree. C. to 100.degree. C. Otherwise, solvent blending can be used."
U.S. Pat. No. 4,864,054 issued Sept. 5, 1989 to J. V. Crivello, et al. describes 1-propenyl aromatic vinyl ethers, including various Bisphenol A-based compounds. The patent teaches in Example I (column 8, lines 45 et seq.) to form a bisallylether of Bisphenol-A by reacting bisethoxylated Bisphenol-A with allylchloride in the presence of sodium hydroxide and tetrabutylammonium bromide.
U.S. Pat. No. 4,518,788 issued May 21, 1985 to J. V. Crivello discloses aromatic polyvinylethers comprising vinyloxyalkoxy substituents on an aromatic moiety, and reaction products of same with active hydrogen compounds such as polycarboxylic acids, phenols, thiols, silanes, and polyols. Compositions comprising such vinyl ether compounds are rendered heat-curable by a thermal curing catalyst comprising an onium salt in combination with an organic oxidant or an aromatic polyvinylether-soluble copper compound.
U.S. Pat. No. 4,705,887 issued Nov. 10, 1987 to J. V. Crivello stands in divisional relationship to U.S. Pat. No. 4,518,788, discussed above. The -887 patent claims the reaction products of the aromatic polvinylether compounds (claimed in the -788 patent per se) with active hydrogen compounds of various specified types.
U.S. Pat. No. 4,617,238 issued Oct. 14, 1986 to J. V. Crivello, et al. describes the formation of photo-curable vinyloxy-functional polysiloxanes by hydrosilation of compounds containing both allyl and vinyl ether functional groups. In the paragraph bridging columns 4 and 5 of the patent, it is taught to form the allyl vinyl ether reactant by adding vinyloxy functionality to an ally-functional compound. Preferred allyl-functional compounds are allyl phenols, with eugenol being most preferred. Column 5, lines 25-39 discloses the reaction of eugenol with 2-chloroethyl vinyl ether, to form an allyl vinyl ether compound wherein the benzene ring is substituted by allyl, methoxy, and vinyloxyethoxy substituents. Such allyl vinyl ether then is reacted with the silane to form the product vinyloxy-functional polysiloxane product. This reaction is said to be based on the discovery that the rate of silane addition to allyl vinyl ethers is much faster at the allyl site than at the vinyl site (column 3, lines 58-61). See Examples 5-6 of the patent, relating to the synthesis of allyl- and vinyloxyethoxy-substituted benzene compounds.
U.S. Pat. No. 4,442,197 issued Apr. 10, 1984 to J. V. Crivello, et al. describes UV photocurable, cationically polymerizable compositions including a cationically polymerizable organic material and a photoinitiator selected from a dialkylphenacyl sulfonium salt of specified formula, sensitized with an effective amount of a polynuclear aromatic hydrocarbon and/or phenothiazine, or a hydroxyaryldialkyl sulfonium salt of specified formula, sensitized with an effective amount of an aromatic ketone. Among the cationically polymerizable organic materials which are disclosed to be useful in the curable compositions described in this patent are vinyl organic prepolymers including multifunctional vinyl ethers (see, generally, column 4, line 55 to column 7, line 40). Among the polyvinyl compounds disclosed (at column 6) in the patent are Bisphenol A divinyl ethers.
U.S. Pat. No. 3,933,509 issued Jan. 20, 1976 to Y. Noguchi, et al. describes a photopolymerizable composition comprising a cationically polymerizable substance and a photopolymerization initiator comprising at least one inorganic or organic acid salt of an indolinobenzospiropyran of specified formula. As the cationically polymerizable substance, the patent describes vinyl ethers and N-vinyl compounds as preferred materials. The patent also discloses poly-functional vinyl ethers as being useful alone or in combination with a mono-functional monomer, citing 2,2-bis-(3',5'-dibromo-4'-vinyloxyethoxyphenyl) propane and 2,2-bis-(p-vinyloxy- ethoxyphenyl) propane as poly-functional species (column 6, lines 48-60). At column 6, lines 67-68, the patent discloses that "[t]he cation polymerizable compounds can be used alone or in a combination of two or more."
U.S. Pat. No. 4,749,807 issued June 7, 1988 to S. C. Lapin, et al. describes vinyl ether-terminated ester oligomers which are cationically polymerizable by radiation in the presence of an onium salt. The vinyl ether compounds disclosed in this patent have one or more terminal vinyl ether groups and are oligomeric esters of carboxylic acids which contain oxycarbonyl linkages in the backbone structure. At column 4, lines 18-22, the patent teaches the reaction of a vinyl ether terminated alcohol, a second polyol, and carboxylic acid, wherein the polyol acts as a chain extender by ester formation with the carboxylic acid.
The vinyl ether polyester compounds disclosed in Examples I and III of the patent are aromatic liquids.
U.S. Pat. No. 4,751,273 issued June 15, 1988 to S. C. Lapin, et al. discloses to form vinyl ether terminated urethane resins, which are produced by reacting acetylene and a glycol with a diisocyanate. The patent discloses at column 6, lines 24-28 that the resins of the patent may be cured by UV radiation in the presence of an aryl-onium salt.
U.S. Pat. No. 4,230,814 issued Oct. 28, 1980 to J. V. Crivello describes heat curable compositions comprising a cationically polymerizable organic material which is thermally cured with an organic oxidant such as an organic peroxide, in combination with an effective amount of hydroxyaryldialkyl sulfonium salts of specified formula. At column 4, lines 16-20, various classes of cationically polymerizable organic materials which can be used in the composition of the patent are disclosed, including vinyl organic prepolymers, such as the vinyl ether compounds described in the paragraph bridging columns 5 and 6 of the patent.
Gallucci, R. R., et al, "Synthesis of Bis(aryloxyethyl) Vinyl ethers via Phase-Transfer-Catalyzed Nucleophilic Displacement on 2-Chloroethyl Vinyl ether," J. Org. Chem., Vol. 48, No. 3, 1983, 342-346, describes the preparation of bis(aryloxyethyl) vinyl ethers, using sodium hydroxide, bis(phenols), and 2-chloroethyl vinyl ether, with a tetraalkylammonium salt phase-transfer catalyst Reaction in dimethyl sulfoxide solution is described, as well as reaction in toluene, and n-butyl alcohol. In the initial work described in this article, neat chloroethyl vinyl ether was employed as solvent (page 345). The article reports that attempts to use potassium hydroxide, lithium hydroxide, or sodium carbonate to prepare product were unsuccessful, and that potassium hydroxide gave some desired Bisphenol-A divinyl ether product but appeared to react preferentially with the chloroethyl vinyl ether component.
Neckers, D. C., "An Introduction to Stereolithography," The Spectrum, Vol. 2, Issue 4, Winter 1989, pages 1 and 6-10, describes stereolithography as a process for forming three-dimensional parts via photopolymerization from a photosensitive monomer such as an acrylate. On page 9, the article identifies a number of shortcomings of acrylates and posits that entirely new polymeric systems and initiation systems are in order. There follows a discussion of the work carried out by J. V. Crivello in developing photosensitive salts which were used as cationic onium photoinitiators, including aryl iodonium salts capable of producing protic acids when irradiated, for polymerizing "a living polymer from an ether or epoxide."
Accordingly, it is an object of the present invention to provide novel aromatic divinyl ether compounds and compositions, which are readily synthesized, have melting points below 25.degree. C., and are amenable to usage in applications such as coatings, adhesives, and sealants, wherein flowable materials are desired.
Other objects and advantages will be more fully apparent from the ensuing disclosure and appended claims.